T. Vidrih1 and A. Hopkins2
1 Biotehnical Faculty, Agronomy Department, Jamnikarjeva 101, 61000 Ljubljana, Slovenia
2 Institute of Grassland and Environmental Research, North Wyke, Okehampton, Devon, EX20 2SB, UK
The act of compaction
Soil bulk density
Effect of compaction on root growth
Throughout history the soil environment has been manipulated for increasing plant production by manuring and soil tillage, but with limited consideration of plant roots. Renewed interest in grass/white clover systems may result in compacted soils on intensively managed pastures because of high stocking rates and long grazing seasons. But white clover growth and efficient N2-fixation require a relatively stable set of soil physical and chemical conditions. Organic matter raises the ability of soil to resist compaction through better aggregate stability. Compaction probably changes the soil environment in more ways that stress sward growth than any other single factor. Livestock trampling decreases porosity and, when compaction changes pore size distribution, vital soil functions are altered. Changes in bulk density are only sensitive to changes in total pore space and do not reflect changes in pore size distribution. Small reductions in pore size greatly diminish the movement of gas through it. Accumulation of other gases in the soil affects the processes that supply nutrients to roots. Persistence of white clover in pasture is directly linked to performance of the roots, which are controlled by microsite conditions that could be suddenly changed through the impact of grazing animals or of wheeled traffic in the case of mown swards.
The management of crops and forages for maximum productivity has been evolving since farming began, perhaps none more so than in the area of intensive grassland farming. In the past, emphasis has been placed on high productivity and growth rates. Plant breeding programmes have, in the main, been conducted under conditions of long regrowth intervals in order to demonstrate their genetic potential. Under grazing, such species are unable to exhibit sufficient plasticity in reduction of organ size to reach the equilibrium required under frequent defoliation (Brock and Hay, 1993) or to grow on compacted soils. The principles of defoliation management for maximum production of quality herbage are well researched, but the impact of frequency and severity of stock treading (trampling, poaching) on the various levels of ecological organization in pasture is not fully understood or its interpretation appreciated. It is well known that it is possible to grow high yields of crops hydroponically, without any soil at all, so long as all the necessary nutrients are supplied in a form acceptable to the growing plant. But grazing can hardly be done without firm ground, and effects of trampling on the soil environment have to be taken into account when white clover persistence or productivity is to be quantified.
During the last three decades there has been a steady increase in stocking rates on grassland which has been made possible by the high rates of N fertilizer applied to intensive pastures. Soil structural and hydrological changes caused by hoof compaction have increased and so have pasture management problems. On crop-land the increased attractiveness of maize, on account of its stable yield and quality, and its relatively simple cropping technique, can lead to even worse deterioration of physical soil properties because of the equipment load (van Dijk et al., 1994). The solution for some of these problems seems to be grass/clover systems which are part of low-input agricultural systems, and these have recently undergone a revival of interest in some temperate-grassland regions. But white clover production, and the efficiency of the N2-fixation process, are both sensitive to changing conditions and soil characteristics and require a relatively stable set of physical and chemical conditions for the symbiotic relationship to function efficiently (McGrath and Jarvis, 1993). In future, the maintenance of persistency and productivity of white clover will be faced with more compacted soils from one or other causes. In the following sections, the present state of knowledge about soil compaction, gaseous diffusion through soil, and root system growth are summarized as far as the literature allows.
During grazing, soil is trodden to a greater or lesser extent and compaction of the soil surface will occur, depending on a number of factors, particularly soil moisture status at the time of grazing (Wilkins and Garwood, 1985). Compaction is a process which leads to increased soil density as a result of the application of stresses, usually of short duration. Harris (1987) reviewed the literature on treading and its effects on white clover growth. This legume is a characteristic species of trodden areas because the treading reduces the shading by other species, or the canopy of the associated species protect white clover stolons from the damaging effect of treading. But, in general, white clover does not resist treading as well as the grasses, though its growth is not always reduced by treading.
In Brittany, Vertes (1989) studied the effects of spring trampling, with or without autumn trampling, on white clover stands, either pure or mixed with perennial ryegrass. In spite of the fact that there was more destruction of clover by trampling in pure stands, the above-ground biomass of trampled plots equalled that of the control plots from the second growth in spring. Differences in stolon biomass persisted until a later date in the year, as did soil structure modifications and changes in the distribution of roots. From another experimental study on pure white clover and on a perennial ryegrass/white clover sward, trampling led to the destruction of a large portion of the aerial system, and induced a large decrease in earthworm density and biomass (Cluzeau et al., 1992). It seems that with better knowledge on how soil physical properties could be changed as a result of trampling, a more accurate explanation of factors affecting persistence and production of white clover could be obtained.
Soils are complex systems with interacting mineral, organic, gaseous, aqueous and biological components. Their structure is defined as the arrangement of single mineral particles and organic substances to greater units known as aggregates and the corresponding inter-aggregate pore system (Horn et al., 1994). Compaction probably changes the soil environment in more ways that stress sward growth than any other single factor. It directly alters four physical properties: bulk density, soil strength, pore size distribution and aeration. Changes to these four physical properties subsequently impact on several soil chemical and biological properties and ultimately affect white clover growth. Many of the changes induced in soils pass unnoticed until too late. Effects on white clover may be delayed until one of these properties becomes limiting in the total soil environment.
Soil bulk density, or volumetric mass, is the weight of solid material in a given volume of soil. Bulk density values typically range between 0.8 and 2.0 g cm-3 depending on soil texture and state of compaction. Bulk density changes are not proportional to the load increase. Each increase in compactive load raises bulk density, but the changes come in smaller increments as the bulk density approaches a maximum. Changes in bulk density are sensitive only to changes in total pore space, so cannot readily reflect changes in pore size distribution.
Soil water content. At the time of compaction this can modify the effect of compactive pressures. Typically, moist soil compacts more than dry or wet soil, and thus white clover is more tolerant of treading in summer than in spring (Edmond, 1964). Some increases in soil bulk density during the season may result in increased soil strength and less poaching as the soil re-wets in autumn. Studies by Scholefield and Hall (1985) show that at or above a critical water content, treading involves some soil compaction, but deep penetration and poaching result from repeated treading under wet conditions. Fine-textured soils are at more risk of poaching than coarse-textured soils. On clay soils, treading will quickly reduce the rate of water infiltration, and will affect white clover growth through reduced soil aeration. In very wet conditions, the soil takes on fluid properties and flows away from a compactive force instead of being compressed (Scholefield et al., 1986). Repeated treading in these conditions produces deep hoofprints, and causes degradation of soil physical properties for a longer period. (Scholefield and Hall, 1986).
Organic matter. There are a number of possible mechanisms by which organic matter may influence the ability of soil to resist compaction such as: (a) binding forces between particles and with aggregates; (b) elasticity; (c) dilution effect; (d) filament effect; (e) effect of electrical charge; (f) effect of friction (Soane, 1990). Humified organic matter has a complex role in stabilizing natural structure and will strongly influence the behaviour of soils. If the organic matter of soils declines, any resulting increase of compactibility under intensive trampling could accentuate a decline in productivity. The dangers of soil degradation if soil organic matter content decline have been reviewed by Newbould (1982). Most likely the organic matter raises aggregate stability and prevents some compression during a compactive load.
Soil strength is a measure of penetration resistance of a soil. In uncompacted soil, penetration resistance typically rises with increases in depth. Clay particles play the most significant role in determining soil strength. The presence of a water film during compaction allows the clay particles to slide and reorient in ways that increase their face-to-face contact. Drying can also increase soil strength by reducing the water film between soil particles so their interparticle attraction increases. Unlike the effect with bulk density, increasing the compactive load continues to increase soil strength significantly. Thus, while a load may not cause any visual deformation indicative of compaction, soil strength could have increased significantly. This feature comes into play especially where repetitive loading occurs. Typically, about 70% of the total compaction occurs with the first compaction event. Subsequent compactions change the bulk density only slightly, but soil strength continues to rise.
Aggregate stability. Soil shear strength increases or decreases with increasing organic-matter content depending on whether or not the form of organic material involved increases aggregate stability, which is a measure of a soil aggregate's resistance to breakdown. Organic matter from grass increases shear strength since it improves the soil aggregate stability, but the increases are limited, since it also reduces bulk density and improves moisture retention, both of which act to reduce soil shear strength. Organic matter from peat reduces soil shear strength since it reduces soil stability. It only adds its bulk to the soil, making the aggregates fall apart, reducing the soil shear strength (Ekwue, 1990). The aggregate formation process usually occurs when soils dry and swell, but also occurs because of biological activities (Wolters, 1991). Soil shrinkage first increases the bulk density of aggregates by forming only a few very wide inter-aggregate cracks. During consecutive swelling and drying the aggregate strength increases and aggregate diameter becomes smaller (Horn et al., 1994). Stable aggregates have been described as a product of temporary mechanical binding by roots, fungal hyphae and short-lived organic adhesives (polysaccharides) produced by microbes in conjunction with long-term cementing actions of more resistant humus components. These aggregates benefit from continual renewal of the polysaccharide component. The most abundant, recurring sources of fresh organic material from polysaccharide production are root exudates and associated debris. While fungi, actinomycetes and bacteria all produce soil aggregates, fungi produce the most (Oades, 1984). Only organisms actively decomposing fresh organic materials in the soil seem to renew the polysaccharide component and encourage stable aggregate formation.
Soil is a porous medium of interconnected openings. Inside these pores roots grow, microbes live, water moves and is stored, and gases flow. Soil in good physical condition typically contains 50% pore space. More important than the total pore space, however, is the pore size distribution. Trampling often decreases porosity in grassland topsoil (Mulholland and Fullen, 1991), but knowledge of how different pore sizes are affected by trampling is limited. The variously sized pores all perform important, but different, roles in the soil, so when compaction changes pore size distribution, vital soil functions are altered. Macropores are the largest soil pores and function in aeration and drainage of excess water. The lower size limit of macropores is defined as 30 m m diameter (Humblin, 1985), but studies of the influence of trampling seldom reach this limit. Mesopores, the medium-sized pores, are most important in distributing water within the soil by capillary action. Capillarity conducts water downward for storage or upward for drying, supplies water to roots and redistributes water from wet to dry zones. Micropores are the smallest soil pores and function primarily as water-storage locations.
The persistence of larger pores depends on the stability of aggregates. The macropore collapse caused by trampling is accompanied by decrease in size of aggregates (Warren et al., 1986; Taboada and Lavado, 1993). It is reasonable to assume that compaction also disrupts continuous pore networks. When this happens, pores become sealed and isolated from each other. Thus, even though pores of optimum size may be present, their ability to function is diminished because of their isolation. Compaction converts macropores into mesopores and micropores. While this change may reduce total porosity only slightly, its greatest impact is the change in pore function in the compacted zone. A macropore network at the soil surface that once facilitated soil aeration can be compacted into a series of unconnected micropores. That change in pore size distribution shifts the function of the soil surface from aeration to water storage, blocks gas exchange between soil and atmosphere and generally degrades the soil's ability to support white clover growth. Water-storage pores need to be located in the root zones, not at the soil surface. Previously damaged soil pores can be regenerated by shrink-swell during wetting-drying cycles (Dexter, 1988). The regeneration mechanism depends on formation of microcracks as a compacted soil layer dries.
Roots of sward plants have a complex role in the mechanical properties of soil. They are acting as a physical network, and are an active source of organic exudates which are likely to be effective agents in stabilizing aggregates (Reid and Goss, 1981). There are also complex interrelationships between roots and the hyphae of mycorrhizal fungi. Thus, conditions that encourage actively growing roots and fungi will enhance aggregate stability and provide some natural resistance to compaction. However, once compaction does limit soil aeration, fungal activity is one of the first to be diminished because of these organisms' low tolerance to anaerobic conditions. In order for natural processes to remedy compaction, fungal decomposition of organic materials should be encouraged, but that is unlikely to occur because compaction creates anaerobic conditions.
The aggregates will give good aeration for root and micro-organism respiration, which in turn allows full expansion of roots within soil to search for nutrients and water. Good root development may allow maximum production at reduced fertility levels, or stabilize pasture production in a variable environment. Such conditions can be found very often on grassland. Larger root systems may contribute to an increased nutrient use efficiency, thus minimizing the likelihood of nitrogen being leached from pasture soils to affect other parts of the environment adversely.
Normal respiration by sward roots and soil microbes depletes O2 content and increases CO2 content. Enhancement of gas transport in the soil occurs both as a mass flow along a pressure gradient and as a diffusional flow with a concentration gradient in air-filled pores (Gupta et al., 1989; Hodgson and MacLeod, 1989). Gas diffusion rates are highly dependent on the cross-sectional area available for diffusion. A small reduction in pore size greatly diminishes the movement of gas through it. Thus, loss of aeration porosity (i.e. macropores) translates directly into reduced soil aeration. The smaller pores that result from compaction are more likely to be blocked by water films. Both conditions - fewer macropores and more blocked passageways - accelerate formation of anaerobic soils, as characterized by a deficiency of oxygen (Currie, 1984).
A useful index of soil aeration is the oxygen diffusion rate (ODR) because of its close correlation with sward growth. ODR values below 0.2 m g cm-2 min-1 are limiting, and values above 0.4 m g cm-2 min-1 are suggested as being adequate (O'Neil and Carrow, 1983). Attempts to model the relationship between soil porosity and gas diffusion through soil show best agreement with a fourth-degree exponential function (Currie, 1984). This means that if compaction reduces air-filled pores to half their original value, gases will then diffuse through the soil 16 times slower than before compaction. Because the soil pores remaining after compaction will be smaller, the likelihood of passages being blocked with water increases. This further reduces gas diffusion since gases like O2 and CO2 diffuse about 64 times slower through a water film than in an open pore (Currie, 1984). With greater root development air-filled pores become occupied by roots. Root-filled porosities are defined as the ratio of the volume of soil pores occupied by roots to the total soil volume at a given water potential. Higher values for pore plugging significantly decrease the ODR values (Asady and Smucker, 1989). Increased respiration rate per unit root length in impeded roots growing in smaller pores (compaction) is responsible for increased oxygen uptake. This causes a local hypoxia, and the whole carbon metabolism of the elongating zone can be disrupted, with possible effects on growth. Oxygen can also reach roots by diffusing down through the intercellular spaces of root tissue. Root porosity, however, decreases in compacted soil (Schumacher and Smucker, 1981). It is believed that compaction distorts root cells sufficiently to compress the intercellular spaces and limit O2 diffusion (Barley and Greacen, 1967).
Besides affecting O2 and CO2 levels, compaction can cause other gaseous by-products to accumulate in soils. It has been shown that when compaction causes O2 to drop below 1%, ethylene (C2H4) rises as high as 20 ppm (Smith and Restall, 1971). As the ethylene level in the soil increases, micro-organisms dependent on oxygen will be inactivated thus regulating the rate of turnover of organic matter, and the insoluble ferric salts are reduced to a ferrous state. This way cationic plant nutrients are released into the soil solution. If this process takes place close to the plant root hairs then the nutrients are in exactly the right place for plant uptake. In well-ventilated soil with macropores aerobic activity will result in a rise in oxygen levels, and the ethylene level will drop. Once the aerobic activity starts, the ferrous iron in solution is oxidized and the unused plant nutrients revert to the insoluble form and leaching is prevented. This oxygen-ethylene cycle will be repeated continuously as long as the soil conditions are favourable, and it is of great importance for nutrient losses from soil. Ethylene gas is always found in undisturbed grassland soils, but in cultivated soils the level is low or non-existent.
Most of the results on the effects of soil compaction reported in the literature are derived from research on crop fields (cultivated land), where attempts are made to maintain desirable soil architecture artificially with frequent soil disturbance. This way a high level of soil aeration and fast mineralization of plant residues in soil are achieved. The duration of research work done on grassland is also often too short. The main results on effects of soil compaction on root systems growth are obtained before the restoration to undisturbed soil is achieved that allows the ethylene-oxygen cycle to reassert itself. There is a lack of results on persistence and productivity of white clover under natural conditions in undisturbed land.
Compaction affects sward growth in many ways and these effects are not always consistent. Even superficial observations show that root systems are less developed in compacted than in non-compacted fields. Great mechanical resistance and the decreased aeration of compacted soils limit both the length and function of sward roots. Elongation rate of individual roots is reduced, while root diameter and branching are usually increased (Scholefield and Hall, 1985). A straightforward interpretation could be that this reduction in root growth is a direct consequence of increased soil mechanical impedance. In contrast to the penetrometer, roots are flexible organs which can follow pores and cracks, they can excrete mucilage and root tips experience almost no friction resistance. Several experimental results cited in the article of Tardieu (1994) suggest that the roots are sensors of local physical conditions in soil via several types of messages. In the case of any stress, not only mechanical impedance, root elongation rate will slow down and the length of the elongation zone is reduced. Both the elongation rate of impeded roots, and the growth of non-impeded roots, are reduced. Goss and Russell (1980) have observed that the rate of root elongation rate remains reduced for 3-10 days, and after that the increased mechanical impedance is released. The existence of a lag-time has been confirmed by Bengough and Young (1993). Roots which had been impeded in a compact layer still elongated at a reduced rate when they left this layer towards a subsequent layer with low mechanical impedance. This process is probably under the control of hormones such as abscisic acid, or ethylene. An increase in ethylene production in impeded roots has been observed by Kays et al. (1974). While compaction is a physical process, the reduced aeration it causes can affect chemical and physiological processes that ultimately impact on plant nutrition. When compaction reduces the O2 supply in soils, an important soil chemical property, called the oxidation-reduction (redox) potential, declines.
Besides affecting nutrient availability, compaction interferes with the processes that supply nutrients to roots. Diffusion of ions through the water film in soil is a primary supply mechanism for many plant nutrients, especially phosphate and potassium. Diffusion occurs most rapidly through continuous water film with a large cross section. In loose soils, compaction can sometimes increase the rate of nutrient diffusion by creating thicker, more continuous water films. The plant nutrients N, P, K, Ca, Mg, S, Zn, Cu, Mn, Fe, Mo, Co and B should be present in the soil to match white clover requirements. Some of these minerals are involved directly in nodulation or N2-fixation processes.
On intensively managed farmland, lack of plant nutrients will not be the main limitation to white clover growth and N-fixation. Increasing evidence suggests that particular parts of the soil-plant system are very sensitive to changes in soil pollutants such as (1) heavy metals, originating from feed supplements, fertilizers, fallout from the atmosphere, and the use of sewage sludge and farm wastes on land; and (2) organically based pollutants, originating from pesticide applications. Thus, the N2-fixation capacity of white clover can be decreased because of an effect on the survival of the bacterial symbiont (Rhizobium leguminosarum biovar trifolii) in the soil (McGrath et al., 1988). These inputs will be smaller on grassland than on land used for arable crops, but resulting concentrations will be greater in the top soil layer of grassland than on periodically ploughed crop-land, because of mixing with lower horizons (McGrath and Jarvis, 1993). Performance of white clover is further dependent on the microbial processes in soil such as residue decomposition, mineralization-immobilization, denitrification, nitrification and nutrient cycling, driven by interactions among soil organisms, and soil architecture. McGill et al. (1993) proposed that, because microbial communities are the components of grassland systems having the shortest life histories and hence the quickest response to perturbation, knowledge of microbial communities in situ may be useful to evaluate environmental stress on soil in grassland ecosystems.
Throughout history the soil environment has been manipulated by manuring, soil tillage, drainage and irrigation for increasing plant production. The great rise in pasture productivity during recent decades has been based to a considerable extent on further modifications of the soil environment, mostly guided by practical experience about root growth and functions. Present day discussions on soil quality often concentrate on manipulation of the soil environment but with limited consideration of plant roots.
Performance in the longer term of grass/clover pastures as a part of low-input agricultural systems will be achieved if knowledge on nutrient cycling, as driven by interactions among soil organisms, and soil architecture are considered when developing new cultivars and methods of farming. Effects of grazing animals are important in the evaluation of white clover cultivars and the yield of DM should not be the major criterion for selection of cultivars for farm systems. In order to allow sustainable pastoral systems to be put in place, this should also include the ability to withstand various forms of grazing, and periods of adverse climatic stress. Trampling and soil compaction have great effects on pore size distribution, particularly macro-aggregate porosity, hydraulic conductivity and gaseous diffusion. These properties may be most important indicators of changes in soil environment that have great relevance for white clover persistency. Most white clover cultivars in use have been selected for high harvest indices and for growing on frequently cultivated land. The persistency of white clover has, possibly, been lost for the sake of productivity. Vegetative propagation through clonal growth processes is the chief mechanism of white clover persistence. There is regular 1-2 years turnover of stolen units and associated roots. In this situation, it is not the death of stolon units that is viewed as persistence but the proportion of new plants of white clover to be successfully established in a desirable soil environment from year to year. There is not much chance of solving soil degradation problems with genetic engineering. We have to realize that any gain in persistency in white clover may be at the expense of its productivity, but we should be aware that 'as the root system goes, so goes the grass/white clover sward'.
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